Digital optical spatial switches (DOS) are well known in the art. One class of such switches is the 1×2 digital optical switch known as a “Y-branch digital optical switch” (Y-branch DOS) wherein light input into the “base” or trunk of the Y is directed through one or the other of the output branches by virtue of changes effected in the refractive index of one or both of the output branches. The switch can also be operated in reverse, whereby one or the other “upper branches” of the Y can be selected as an input channel with the base of the Y being the output channel. Y-branches are a fundamental building block of optical circuitry, and may be employed singly, or in various combinations to form more complex switching and coupling devices.
The Y-branch DOS has received wide commercial acceptance primarily because of its robustness to variations in critical parameters such as electrical power applied, polarization, wavelength, temperature, and to a large extent, even device geometrical variations. Typically a Y-branch DOS is designed such that two waveguide branches intersect to define a Y-shape structure with a very small angle at the intersection of the branches. The composition of the waveguide structure may include a wide variety of materials such as lithium niobate, semiconductors, silica, or polymers. A Y-branch DOS performs its switching function by adiabatically changing (i.e. slowly varying, as opposed to abruptly altering) the light propagation direction in one of the output waveguides.
Specifically, switching in a Y-branch DOS is achieved by forcing a refractive index change in one waveguide branch with respect to the other. The change in refractive index may be induced by applying for example voltage and/or current to selected sections of the structure. Of particular significance among the characteristics of a Y-branch DOS is its step-like response to applied voltage or current, which allow the light to remain in a higher index branch, notwithstanding an increase in the applied voltage or current beyond the switching threshold. When a Y-branch DOS operates above the switching threshold, variations in polarization and wavelength do not impact significantly the switching capacity of the Y-branch DOS.
One persistent problem presented to the designer by the Y-branches of the art is footprint. In order to effect the adiabatic transfer of energy, known in the art as adiabatic modal transfer (AMT), of the propagating wave into the single output channel selected, it is necessary to maintain a separation of the two output branches of no greater than ca. 30 times the wavelength of the propagating signal for a silica-fiber-level of refractive index difference in the waveguide of ca. 0.5% of the base index. For 1.5-micron radiation, this means that the separation between the two branches must be maintained at a distance on the order of 45 micrometers or less until the energy transfer is complete. This requirement in turn necessitates very small vertex angles on the order of 0.1 to 0.3 degrees and device lengths up to 30 mm. Controlled fabrication of such devices is quite difficult and error prone. Furthermore, the large footprint of such devices greatly limits their applicability in integrated optical circuitry.
One approach to addressing these problems is provided by Okayama et al, J. Lightwave Tech. 11 (2), 379-387 (1983), in which a two angle shaped Y-branch DOS wherein the output waveguides initially diverge by an angle of ca. 2° and then undergo a bend to a smaller extrapolated angle of divergence of ca. 0.3°.
Several methods are known in the art for effecting the desired change in refractive index. These involve the electro-optic effect, the stress-optic effect and the thermo-optic effect. In a typical Y-branch thermo-optic DOS known in the art, the two upper branches of the “Y” are provided with a heating means, typically a thin layer of metal deposited thereupon, which heating means when activated induces a shift in the refractive index of the corresponding branch, thereby effecting a coupling of power input to the base of the “Y” to one or the other branches. By turning on the heating of one branch and turning off the heating of the other branch, switching of incoming optical signals can be effected.
Both polymeric and glass Y-branches are known. Because of the much larger temperature dependence of the refractive index of polymers, polymers are preferred for use in thermo-optic digital optical switches.
Hida et al, IEEE Photonics Technology Letters 5 (7), 782-784 (1997), disclose polymeric 2×2 thermo-optic switches consisting of two coupled Y-branches fabricated from deuterated and fluoro-deuterated methacrylate polymers. The method of fabrication involves spin coating polymer solutions onto a silicon substrate followed by forming the Y-shaped components by conventional photolithography, the core ridges being subsequently formed by reactive ion etching. Chromium thin-film strip heaters were formed on the upper Y-branches by electron beam evaporation and wet etching. Separation of the arms was 250 micrometers. The Cr heater strips were 5 mm long and 50 micrometers wide.
Eldada et al, Proc. SPIE, vol. 3950, pp. 78-89 (2000), discloses 1×2 optical switches fabricated from polymeric materials which Y-branches exhibit 0.1 dB insertion loss for vertex angles of less than 2°. The direct photolithographic fabrication method using halogenated acrylates as practiced therein is disclosed to enable sharp profiles of the components and the removal of residue even at the vertex of relatively small angle Y-branches.
Lackritz et al, U.S. Pat. No. 6,236,774B1, discloses thermo-optic switches employing cross-linked polymeric waveguides operated above Tg. Disclosed are metallic heaters substantially rectangular in shape disposed upon a polymeric optical waveguide surface, the long side of said rectangular heater being positioned at a slight angle to the direction of propagation in the waveguide. Said heaters are positioned to be in uniform thermal contact with waveguide material over the entire area of the heater. It is disclosed that the temperature, and therefore the refractive index, of the polymer waveguide material will depend upon the distance of any point therein from the heater, those regions closest to the heater experiencing greater temperature than those further away.
He et al, U.S. Pat. No. 6,526,193 B1, discloses the electro-optic effect in a Y-branch digital optical switch having curved output waveguides provided with curved electrodes to provide a shorter device than achievable in the earlier art which employed diverging straight waveguide sections. The curvature of the output waveguides provides a continuously increasing angle of divergence.
Lee et al, U.S. Pat. No. 5,623,566, is drawn to thermally induced guides in silicon optical benches. Disclosed in FIG. 2 thereof is the temperature profile through the various optical materials employed therein as a result of localized heating applied thereto.
Moosburger et al, Proc. 21st Eur. Conf. on Opt. Comm, pp. 1063-1067 (1995) disclose Y-branches with “near perfect” vertices having an angle of 0.12° fabricated from silica-clad polymeric waveguides having cores of ca. 9 micrometers. The upper branches of the Y were coated with Ti thin film heaters. 27 dB cross-talk suppression was achieved between output branches with heater power of ca. 180 mW. Moosburger expressly teaches that blunted vertices induce losses and reduce the crosstalk suppression between output waveguides.
Diemeer, Optical Materials 9, 192-200 (1998) provides a thoroughgoing analysis of the thermal transport and physical aspects of thermo-optic switching in polymeric vs. silica thermo-optic digital optical switches. For polymers in general, and polycarbonate and polymethylmethacrylate in particular, it is shown that switching power lies in the range of 50-100 mW, and that a temperature rise of ca. 10° C. in the waveguide core is necessary to achieve a minimum refractive index difference of ca. 0.001.